Technologies for detecting defects in a steel plate may include an ultrasonic test, a magnetic flux leakage inspection, a magnetic particle inspection, an eddy-current inspection, an optical inspection method and the like.
Among these, a magnetic flux leakage inspection is a scheme of detecting a portion of magnetic flux leaked to the outside of a steel plate due to defects present in a steel plate when the steel plate is magnetized in a certain direction, using a magnetic sensor or a hall sensor. Such a magnetic flux leakage inspection may have superior performance in terms of the detection of crack defects occurring in a surface or below a surface layer of a ferromagnetic metal, and an example of an inspection device using the magnetic flux leakage described above is disclosed in Cited Document (Korean Patent Laid-Open Publication No. 2010-0076838).
Referring to FIG. 1 in connection with Korean Patent Laid-Open Publication No. 2010-0076838, a magnetizer 120 may be disposed on an upper portion of a steel plate 10 wound around a roll 110. Electromagnetic poles 121a and 121b are alternately arranged in a lower region of the magnetizer 120 and a coil 122 is wound on an upper region of the magnetizer 120. Here, in a case in which currents flow in the electromagnetic poles 121a and 121b through the coil 122 in opposite directions, the steel plate 10 may be magnetized in a direction from the electromagnetic pole 121a having N-polarity to the electromagnetic pole 121b having S-polarity. In this case, a leakage magnetic flux may be sensed by a sensor 131 disposed between the electromagnetic poles 121a and 121b, such that defects present in the steel plate 10 may be detected. Meanwhile, the electromagnetic poles 121a and 121b may be spaced apart from each other by a predetermined interval and be configured such that they are inclined at a certain angle (θ) with respect to a rolling direction and thus, are formed to a boundary line A at which the steel plate 10 comes into contact with the roll 110, in a spiral manner.
However, according to the Cited Document, the following limitations are present.
First, as illustrated in FIG. 2, accurate detection of defects may be difficult due to a non-detection region R1 in which magnetic flux is unable to be detected. In magnetic flux formed in ends of the electromagnetic pole 121a having N-polarity and the electromagnetic pole 121b having S-polarity, directions and degrees of intensity thereof may not be uniformly formed (see reference numeral 201 and R2).
Second, as illustrated in FIG. 1, the magnetizer 120 may have an integrated structure in which a plurality of the electromagnetic poles 121a and 121b may be integrally provided. Therefore, it may be unfeasible to replace only some units such as the electromagnetic poles 121a and 121b or the sensor 131, leading to difficulties in terms of maintenance and management.
Third, since defects present in a surface and an interior portion of the steel plate 10 may be simultaneously detected, it may be difficult to separately detect a surface defect and an interior defect or accurately determining a position of the interior defect may be unfeasible.